1. Field of the Invention
The invention relates to a heat-insulating spray coating film formed on a surface of an aluminum substrate, an engine having the spray coating film, and a film-forming method of the spray coating film.
2. Description of Related Art
For preventing heat from transferring to aluminum substrates, spray coating films with a heat insulating effect have always been formed on surfaces thereof. As the parts requiring such a heat insulating effect, for example, an engine can be mentioned.
In an engine, fuels combust in the combustion chamber, thus it is desired that combustion heat would not dissipate from the combustion chamber in order to improve combustion efficiency. What is important is to decrease the thermal conductivity of wall surfaces of the combustion chamber, i.e. to endow the wall surfaces thereof with a heat insulating effect.
In view of such a problem, for example, Publication of Japanese Patent No. 2013-185200 suggests a spray coating film having a first spray coating film formed on a surface of an aluminum substrate and a second spray coating film formed on a surface of the first spray coating film. The first spray coating film is a film composed of Ni—Cr alloy (Ni-based alloy), and the second spray coating film is a film in which SiO-based oxide is filled in pores of a sprayed porous oxide film including ZrO2-containing particles.
In accordance with the spray coating film, heat insulation can be improved by including ZrO2-containing particles in the second spray coating film. In addition, by filling the SiO-based oxide between the ZrO2-containing particles, penetration of fuels into the second spray coating film can be prevented.
Nevertheless, in Publication of Japanese Patent No. 2013-185200, thermal conductivity of the second spray coating film is reduced by containing ZrO2 in the second spray coating film, thereby the heat insulation of the second spray coating film is ensured. However, volumetric heat capacity of the second spray coating film would not be lowered sufficiently by only filling SiO-based oxide between the ZrO2-containing particles. Hence, once the second spray coating film is heated, temperature of the second spray coating film would not be lowered sufficiently.
For example, when such a second spray coating film is formed on wall surfaces of a combustion chamber of an engine, temperature of the second spray coating film which serves as wall surfaces of the combustion chamber would rise immediately and thus explosion may occur, though heat insulation of the combustion chamber is ensured.
Further, in the case of the second spray coating film containing SiO-based oxide, thermal expansion ratio of the second spray coating film is excessively small in comparison with that of aluminum substrate. Consequently, thermal stress is generated due to difference in thermal expansion from the aluminum substrate. Even if a first spray coating film composed of Ni-based alloy is provided, the thermal stress cannot be absorbed sufficiently by the first spray coating film. As a result, the second spray coating film may peel off.